Plastic film used for wrapping produce such as lettuce is usually perforated to provide ventilation for the produce. One type of perforation which is especially suitable is a pattern of opposed C-shaped cuts. The opposed cuts form butterfly valves which provide the desired degree of ventilation. It would be desirable to form the cuts in the film in a continuous manner using a rotary punch, but it is difficult to form C-shaped cuts in plastic film because the film tends to stretch and tear when struck obliquely with a rotary punch, and the punch must also be withdrawn obliquely without tearing the film. Accordingly, prior to this invention, C-shaped cuts were made in plastic film in a noncontinuous operation in which an array of vertical punches was brought down onto a horizontal, stationary section of film. The maximum speed of the operation was only about 100 feet per minute. The punches were hollow cylinders having serrated cutting edges and two rectangular slits down opposite sides.